The Study On PH Changes And Algae Growth Response Model In Alkaline Lakes Under Water Diversion Conditions

Under the stress of global change and impacted by anthropogenic disturbance,a class of naturally alkaline lakes located in arid-semi-arid regions are mostly facing water levels fall and significant shrinkage of the lake surface.Therefore,water diversion from outer basins for water recharge is an important measure to maintain the lake ecosystem.However,the alkalinisation of alkaline lakes is actually a long-term process.Under the influence of transfer of water from external basins in a short time,the influx of neutral river water from the external will lead to a sudden drop in lake pH.pH is a key element in maintaining the structural stability of the special aquatic ecosystems of alkaline lakes,and it also plays a decisive role in the growth of algae and their community composition.A decrease in pH may have irreversible effects on the unique algal communities that have already adapted to their alkaline environment,which will lead to a series of unknown and uncertain ecological risks.Therefore,it is urgent to study the effects of the water diversion from outer basins on pH changes and algal ecological processes in alkaline lakes,so as to protect the ecological environment for alkaline lakes.Thus,the development of a coupled hydrodynamic-ecological model of algal growth in response to pH changes can provide an effective tool for such alkaline waters or similar waters when carrying out ecological management and development.This paper takes the Chenghai Lake,one of typical alkaline lakes in the Yunnan-Guizhou plateau of China as an example,and systematically summarizes structural characteristics of algal communities,temporal and spatial distribution patterns as well as their influencing factors.By an experimental study of mechanism,the influence of pH change on the growing pattern of algal communities in alkaline lakes was revealed,and the limiting function of pH on the growth of cyanobacteria and chlorophyta is proposed.Additionally,based on the carbonic acid equilibrium theory,a set of pH calculation model was proposed for the mixing of rivers and lakes.An ecological model,which it was coupled to hydrodynamic model and pH calculation model,was developed.Finally,the model was then applied to Chenghai Lake to simulate the variation of pH and evolution trend of algae under water diversion conditions.The main results are as follows.（1）The population structure of planktonic algae and temporal distribution patterns in the Chenghai Lake,as well as their responses to various habitat factors,were grasped.The results of the historical ecological research of the Chenghai Lake and the monitored data of ecological environment from 2014～2019 were summarised,it was found that pH in Chenghai Lake area is almost stable in the range of 9.21～9.56 all year round,algal blooms in the Chenghai Lake were mainly spotted in the winters and springs from Decembers to Mays.In the community structure of phytoplankton,Cyanobacteria and Chlorophyta are the absolute dominant algae and the secondary dominant algae,respectively,among them,Anabaena and Scenedesmus were the most common.In addition,correlation analysis with Spearman test revealed that phosphorus was the key factor dominating the annual algal bloom pattern in the lake,with algal biomass increasing significantly when the TP concentration exceeded 0.04 mg/L.（2）The pH variation response to the growth patterns of the dominant Cyanobacteria,Anabaena and the dominant Chlorophyta,Scenedesmus and their interspecific competition patterns were investigated.The results showed that both species are basophilic algae and that under the experimental pH conditions（from pH7 to 11）,the growth of both the Anabaena and the Scenedesmus was inhibited in low pH while promoted in high.The Anabaena was more sensitive to pH changes than scenedesmus.In addition,according to the Lotka-Volterra competition model,Anabaena eventually won in alkaline environments at pH≥9,drawing the conclusion that high pH is the vital element causing Anabaena’s dominance in alkaline waters.（3）A pH mathematical model of alkaline lake is constructed.Based on the water ionization equilibrium and carbonic acid equilibrium laws amongH ₂CO₃、HCO₃^-、CO₃^2-and OH^-,a precise,effective and applicable pH calculation method for rivers and lakes mixing was established.On this basis,a three-dimensional prediction model of alkaline lake pH is developed by coupling the hydrodynamic mathematical model.And this model is validated by the field observation data of pH in the pumping test of another typical alkaline lake,Yamdrok Lake.The validation also indicated that the model can simulate preferably the spatial dynamic variation of alkaline lakes’pH during neutral river recharge.（4）Based on the pH model,the effect of water diversion on the pH of Chenghai lake area and its cumulative evolution trend over the years were simulated and studied.The results show that the effect of water diversion on pH mainly focused in the northern parts near the inlet in Chenghai Lake.The water diversion with relatively long and uniformly distributed flow has less impact on the pH of the lake area than that with high flow in a short period of time.Water diversion in windless conditions would result in a dramatic increase in both the extent and duration of the impact on the lake pH.So,attention should be paid to wind changes off the lake surface during the transfer,and avoiding transfers in windless periods as much as possible.In addition,over the years,the diversion has a certain cumulative impact on the pH of the lake,with a total reduction of 2.2%after the five-year water level upgrade compared to the pre-transfer pH.（5）Based on the experimental results,a formula describing how pH limit the growth of cyanobacteria and chlorophyta was purposed.And it was added to classical algal ecological dynamics model to improve the model growth equation.A coupled model of hydrodynamic,water quality and ecological with cyanobacteria and chlorophyta as the dominant algal species was also constructed by adding the improved algal growth equation and pH calculation model to this model.The model was then deployed in numerically simulating the algal growth process and water quality in Chenghai Lake from 2014 to 2016,when the water diversion was not conducted.After validation by the measured data,it was concluded that this model can preferably show the spatial and temporal changes of water temperature,chlorophyll a,pH and various nutrients in Chenghai Lake,and can grasp the overall trend pattern of algal biomass,as well as capture the algal growth process in the lake.The model can also effectively predict the algal ecology and evolution of the kind of alkaline lakes studied in this paper,and can provide a useful tool for subsequent ecological management of similar waters.（6）Using the coupling mathematical model,the algal evolution trend of Chenghai Lake under water diversion scenarios was simulated and predicted.The simulated results show that the massive influx of neutral high-quality river water will lead to a decrease in the algal biomass level and cyanobacterial biomass proportion in the whole lake area,which will drop by about 15.4%～33.8%and 21.9%～37.5%respectively after the completion of the multi-year water level upgrade.The cumulative effect of the water diversion over many years has led to a change in the intra-annual variation of algal biomass from a’multi-peak’pattern with peaks in Marchs,Mays and Augusts before the transfer,to a’single-peak’pattern with a peak in Mays only.The intra-annual variation in the proportion of cyanobacteria and chlorophyta also changed from a’double-peak’pattern with peaks in Mays and Septembers to a’single-peak’pattern with peaks in Mays only after the transfer.In addition,the effect on algae is less pronounced when the flow is evenly distributed over a long period than when the flow is high over a short period.The water diversion with relatively long and uniformly distributed flow has less impact on the algae of the lake area than that with high flow in a short period of time.The reduction in pH caused by the transfer also exacerbates the decline in algal biomass and cyanobacterial proportion in the lake,with the contribution up to 13.8%and17.6%respectively.While under extreme windless conditions,the reduction in pH caused by the transfer may cause chlorophyta to replace cyanobacteria as the dominant algae on a large extent of the lake,the possible risk of ecological structure succession is worthy of attention.